1. Field of the Invention
This invention relates to a resinous composition for sliding members having good frictional and abrasion-resistance properties, and more specifically it provides a resinous composition of polyamide resin which can produce sliding members having high limiting PV value and low coefficient of kineteic friction, and being small in abrasion loss per se as well as of adverse (or opposed) members and further being excellent in rigidity.
2. Description of the Prior Art
Polyamide resin has many excellent properties required for engineering plastics such as toughness, heat-resistance, chemical resistance, etc., although it has some defects such as a loss in mechanical strength, and dimensional stability when moisture is absorbed. Furthermore, this resin is better in self-lubricity than other engineering plastics. From these merits, this resin is useful for parts of bearings, gears and other parts of many kinds of machines which require abrasion-resistance and as a metal substitute because such parts constructed by this resin can be used without lubricant and generate low noise under sliding.
Recently, the fields of use of plastic sliding members is expanding to bearings under large loading without lubricant, bushs at high atmospheric temperature and several sliding parts having thin wall derived from minizing of the machines and their members or parts. Under such situation, the properties required for plastic sliding members become severe.
Generally speaking, plastic materials are good in self-lubricity, but are low in limiting PV value and worse in mechanical strength such as rigidity, when compared with metalic materials. Besides, the term "limiting PV value" can be called as "limit of PV" or "PV limit", in which P is loading pressure and V is peripheral velocity. In other words, a limiting PV value means a minimum value of P x V where a sliding member rotating at a peripheral velocity and loading pressure is melted or seizured by heat.
In general, in order to use plastic materials for sliding members such as bearings, the materials are desired to be not only low in coefficient of kineteic friction, high in limiting PV value, low in abrasion loss, good in frictional characteristics such as less damage for adverse materials, but also be superior in the rigidity and creep-resistance.
Along this line, several plans for improving mechanical properties and thermal deforming temperature of polyamide resin without influencing its friction and abrasion characteristics or rather improving such characteristics have heretofore been proposed. These proposals are, for example, to combine glass fibers and PTFE or carbon fibers and molybdenum dioxide with polyamide resin. However, these known techniques have the following defects.
Namely, in a bearing made from polyamide resin combined with glass fibers and PTFE, the glass fibers are microscopically exposed as it is used so as to "bite off" the adverse members (such as rotating shaft). Indeed, this "bite off" is a great drawback of this type bearing, and the abrasion loss and the friction coefficient become greater.
On the other hand, another bearing made from polyamide resin combined with carbon fibers and molybdenum disulfide is very expensive because of high cost of carbon fibers or molybdenum disulfide per se. Such bearing also suffers from the afore-mentioned "bite off".
Furthermore, common to the above known bearing materials, the lengths of the glass or carbon fibers are so long as about 3 mm that the molded products have large anisotropy so as to trouble the designation of the molds and moreover to lower the dimensional accuracy of the molded product after annealing.
As the result of the study for improving the defects such as "bite off" and anisotropy in molding by comparing long and hard fibers of glass or carbon in the combination with polyamide resin, the inventors have investigated the use of potassium titanate whiskers (hereinafter called as `PTW`) which are finer fibers than the glass or carbon fibers. At first, the investigation was directed to the combination of the whiskers with polyamide resin. Although this two components composition has improved on the limiting PV value and mechanical properties, the specific abrasion and abrasion resistance became worse all the more. These two characteristics are essential properties for material aimed for sliding members.
Unexpectedly, it has been found that the addition of powdered high-density polyethylene (hereinafter called as `HDPE`) to the above two components composition bring about remarkable improvement on mechanical characteristics, thermal resistance (deforming temperature of the molded product), shrinkages after the molding and further, coefficient of kineteic friction and abrasion loss were lowered as the amount of PTW was increased.